1. Field of the Invention
This invention relates to the field of analog front end circuits, particularly those used to convert an input voltage to a differential current.
2. Description of the Related Art
Circuits that convert an input voltage to a differential current are widely used, especially within RF circuit blocks; these V-to-I conversion circuits are referred to as "analog front end" (AFE) circuits. They are typically implemented using a differential pair of npn transistors which are emitter-coupled, and are biased with a current source connected to the coupled emitter junction; the input stage of a Gilbert mixer is an example.
A known AFE circuit is shown in FIG. 1. The emitters of a differential pair of npn transistors Q1 and Q2 are coupled together at a node 8, and the pair is biased with a current source consisting of an npn transistor Q3, with Q3's collector connected to the node 8, its emitter connected to ground through an emitter resistor R.sub.e, and its base connected a bias voltage V.sub.b. A differential input voltage composed of signals RF+ and RF- is connected across the respective bases of Q1 and Q2, and differential current signals I.sub.out+ and I.sub.out- are produced at their respective collectors. An input resistor R.sub.i is often connected across the differential input voltage lines to provide a particular input impedance to the incoming signal; for example, 50 .OMEGA. is typically presented to an RF input. The differential output current is sometimes converted back to a differential voltage by connecting the collectors of Q1 and Q2 to a supply voltage V.sub.supply via load resistors R.sub.L1 and R.sub.L2, to produce differential output voltage signals V.sub.out+ and V.sub.out-. AFE's of this type are discussed, for example, in Gray and Meyer, Analysis and Design of Analog Integrated Circuits, John Wiley and Sons, Inc. (1984), pp. 194-197.
A problem with the circuit of FIG. 1 is that some of its supply voltage headroom is lost to the current source connected to the coupled-emitter junction. "Headroom" is defined as the portion of a circuit's supply voltage which is available for use by the circuit's signals. Depending on the circuit's design requirements and temperature, the voltage at Q3's emitter V.sub.e is typically between about 200 and 500 mv. Q3's base-emitter voltage V.sub.be is typically between about 700 to 900 mv (for a npn transistor), and its collector-base voltage V.sub.cb is about 0 to 300 mv. Adding these voltages together produces a voltage at node 8 of between about 0.9 and 1.7 volts. Additional voltage is necessarily dropped across Q1 and Q2. If the voltage at node 8 is one volt, and another one volt is dropped across Q1 and Q2, V.sub.out+ and V.sub.out- can only assume voltages between V.sub.supply and 2 volts. This loss of valuable supply voltage headroom can be a significant portion of the total supply voltage, which is often as low as 3 volts. Lost headroom reduces the dynamic range of the AFE and can necessitate the use of a higher voltage power supply, which increases the power dissipation needs of the circuitry using the AFE and subsequently increases the weight of the system employing the circuit.